1. Field of the Invention
The present invention relates to an apparatus for converting color temperature in a color display system and a method thereof, and more particularly to an apparatus for converting color temperature by creating converting factors with a plurality of factor portions to convert a predetermined input color temperature into a predetermined aiming color temperature, and a method thereof.
2. Description of the Related Art
A person generally perceives that color tone varies based on an illumination characteristic, when the person sees something. In other words, while one may perceive that the color tone of something under an incandescent lamp is generally reddish, it is comparatively bluish in daylight. Therefore, in a color display system, there is a need to change the color of an object input in the system, to a color that a person sees. The color tone that is generally shown relates to a color temperature. The color temperature means a temperature of a black body when the color of light sources is the same, measured in unit Kelvin [K]. If the color temperature is high, then it is bluish. If the color temperature is low, then it is reddish.
The color display system is broadly used for apparatuses that need to transmit visual information to a user. The apparatuses include, for example, a TV, a digital TV, a TFT (Thin Film Transistor) monitor, a color printer, a digital camera, a projector, a mobile phone, and so on. The color temperature should be adjusted for improving the quality of a picture in the color display system.
Color models are a RGB (red, green, blue) model, a Hue, Saturation, Brightness/Hue, Lightness, Saturation (HSB/HLS) model, a Munsell color system, and a Commission Internationale de l'Eclairage, or CIE, color model. The CIE color model is decided at International Commission on illumination, which defines lighting standards. The CIE color model includes a CIEXYZ, a CIELTV, and a CIELAB. The CIEXYZ color model expresses RGB values as a set, X, Y, Z, of different values, all having positive signs. Generally, the X, Y, Z are called tri-stimulus.
A conventional method of converting the color temperature uses the CIEXYZ color model, and more particularly, it is a color temperature conversion method using both the X-axis and Y-axis.
From now on, the conventional color temperature conversion method will be described by referring to FIG. 1.
A detailed procedure of the conventional color temperature conversion method has been described on “Color Science: concepts and methods, quantitative data and formula, 2nd edition, pp.145–146, 1982” by Gunter Wyszeck and W. S. Stiles. According to the conventional color temperature conversion method, when an image is input (S10), the color temperature Tc of the input image is calculated (S20). Then, a daylight spectrum related to the color temperature Tc of the calculated input image is calculated (S30). The daylight spectrum is calculated by following mathematical expressions 1) and 2) in accordance with a range of the color temperature Tc of the input image. 1) The color temperature Tc of the input image within a range of 4000K≦Tc<7000K is calculated by,                               X          D                =                                            -              4.067                        ⁢                                          10                9                                            T                C                3                                              +                      2.9678            ⁢                                          10                6                                            T                C                2                                              +                      0.09911            ⁢                                          10                3                                            T                C                                              +          0.244063                                    [Mathematical Expression 1]            
2) The color temperature Tc of the input image within a range of 7000K≦Tc<25000K is calculated by,                              X          D                =                                            -              2.0064                        ⁢                                          10                9                                            T                C                3                                              +                      1.9018            ⁢                                          10                6                                            T                C                2                                              +                      0.24748            ⁢                                          10                3                                            T                C                                              +          0.237040                                    [Mathematical  Expression  2]            YD=−3XD2+2.87XD−0.275
A spectrum radiant power distribution SD (λ) of the daylight is calculated by,SD(λ)=S0(λ)+M1S1(λ)+M2S2(λ)  [Mathematical Expression 3]
where, [S0(λ), S1(λ), S2(λ)] correspond to [sO[31], s1[31], s2[31]], and sO[31], s1[31], s2[31] are fixed values, while       M1    =                            -          1.3515                -                  1.7703          ⁢                      x            D                          +                  5.9114          ⁢                      y            D                                      0.0241        +                  0.2562          ⁢                      x            D                          -                  0.7341          ⁢                      y            D                                    M2    =                  0.0300        -                  31.4424          ⁢                      x            D                          +                  30.0717          ⁢                      y            D                                      0.0241        +                  0.2562          ⁢                      x            D                          -                  0.7341          ⁢                      y            D                              
X, Y, Z of a Macbeth color checker (MCC) is calculated (S40). The MCC obtains the X, Y, Z of each path from the following mathematical expression 4. At this time, the applied MCC has thirty-one (31) elements when a wavelength (λ) is 400 nm to 700 nm. The interval of each element is 10 nm.                                                                                           [                                                                                                              X                          i                                                      M                            ⁢                                                                                                                  ⁢                            E                                                                                                                                                                                        Y                          i                                                      M                            ⁢                                                                                                                  ⁢                            E                                                                                                                                                                                        Z                          i                                                      M                            ⁢                                                                                                                  ⁢                            E                                                                                                                                ]                                                  3                  ⁢                                      ×                                    ⁢                  1                                            ⁢                                                                    =                                    ⁡                                      [                                                                                                                                                      x                              _                                                        ⁡                                                          [                              i                              ]                                                                                                                                                                                                                                      y                              _                                                        ⁡                                                          [                              i                              ]                                                                                                                                                                                                                                      z                              _                                                        ⁡                                                          [                              i                              ]                                                                                                                                            ]                                                                    3                  ⁢                                      ×                                    ⁢                  31                                                                                                                                              [                                          R                      ⁡                                              [                        i                        ]                                                              ]                                                        31                    ⁢                                          ×                                        ⁢                    31                                                  ⁡                                  [                                      E                    ⁡                                          [                      i                      ]                                                        ]                                                            31                ⁢                                  ×                                ⁢                1                                                                        [Mathematical Expression 4]            
where, i=1, 2, . . . , 31,       and    ⁢                  [                                                      x              _                        ⁡                          [              i              ]                                                                                      y              _                        ⁡                          [              i              ]                                                                                      z              _                        ⁡                          [              i              ]                                            ]        3    ×    31  is the function value of color matching, and
[R[i]]31×31 is a refeltance diagonal matrix related to one color path, and each wavelength band value exists only in the diagonal term of the matrix. Off-diagonal terms are all zero, and
[E[i]]31×1 is a spectrum power dispersion of the illumination.
XiME, YiME, and ZiME are MCC tri-stimulus values related to the color temperature Tc of the input image.
The X, Y, Z of the MCC of aiming color temperature is selected (S50).
A converting matrix is calculated (S60) according to the following mathematical expression 5, using correlation of the color temperature Tc of the input image and the aiming color temperature.[TM]3×3=[(XYZ)MT]−13×24[(XYZ)ME]3×24  [Mathematical Expression 5]
where, [TM]3×24 is a converting matrix, and
[(XYZ)MT]−13×24 is a matrix of MCC tri-stimulus values related to the aiming color temperature, and
[(XYZ)ME]3×24 is a matrix of MCC tri-stimulus values related to the color temperature Tc of the input image.
The input image is converted to an image of the aiming color temperature (S70). In other words, it is converted to an image of an aiming illumination by using the calculated converting matrix.                               [                                                                      X                  t                                                                                                      Y                  t                                                                                                      Z                  t                                                              ]                =                              [                          T              ⁢                                                          ⁢              M                        ]                    ⁢                                          [                                                    X                                                                    Y                                                                    Z                                              ]                                    [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          6                ]            
where, Xt, Yt and Zt are the tri-stimulus elements according to the aiming color temperature, and
[TM] is the converting matrix, and
X, Y and Z are the tri-stimulus elements according to the color temperature Tc of the input image.
As described above, the image that is converted to the aiming color temperature is output (S80) to various color display apparatuses.
In the conventional color temperature conversion method, as described above, the tri-stimulus values X, Y, Z of the MCC related to the color temperature Tc of the input image and the tri-stimulus values X, Y, Z related to the aiming color temperature, should be calculated to draw out the color temperature converting matrix. The number of matrices needed to realize the conventional color temperature conversion method in hardware is shown in table one.
TABLE 1Type of MatrixNumber Needed31 × 1 324 × 311 3 × 31124 × 3 1
Also, a matrix converting part is needed to write out the converting matrix. In addition, a plurality of multipliers and dividers are needed for calculating the above mathematical expressions. Therefore, applying the conventional color temperature Tc conversion method in hardware generates a problem of low utility in terms of magnitude and costs.